Bumping can be good or it can be bad. Airline bumping is what happens when a passenger is holding a confirmed ticket for a flight and the airline doesn't let you board. You have to have purchased a ticket and checked in for the flight, either at the gate or up at the check in desks. But if the airline bumps you, it provides travel on a future flight to the same city, and some type of compensation. The compensation is usually a voucher for future travel or a free ticket.
Types of Bumping
Bumping can happen voluntarily or involuntarily. In voluntary bumping, a passenger may see that the flight is full or overbooked and ask to be bumped or to have his or her name put on the bumping list. If a passenger is voluntarily bumped, the airline will typically be offering a voucher for a predefined amount, such as $300. Of course, the passenger would also receive a seat on the next flight to their destination. Many years ago, the vouchers were generally for a full one-way flight, but more recently most airlines provide a monetary voucher which may be less than a full one-way flight, depending on the route.
But bumping also happens involuntarily. That's when the airline denies you boarding, even if you have a confirmed seat. This also only happens in oversold situations, but it happens when no passenger volunteers to give up their seat.
For more information on specific bumping practices, ask the airline you're flying on for their rules and compensation policies for bumping.
How to Get Bumped
One of the most important tips for getting bumped is getting to the airport early. Check in for your flight, then ask the gate agent if your name can be put on a list for bumping, if the flight is in an oversold or full capacity. The second tip is to check back occasionally with the gate agent as it gets closer to departure time. Of course, you are more likely to get bumped from routes that have a high number of passengers, and a high number of business travelers.
Personally, I've had both good and bad experiences with bumping while flying. A number of times, when I've had the time to wait and wasn't in a rush to get somewhere, I've volunteered to give up my seat to earn a free future ticket or voucher for future travel. If this is what you're hoping to do, you usually want to get to the gate early and have your name put on a bumping list by letting the gate agent know you'd be willing to take a later flight. Of course, be careful and check when the next flights might be.
You also want to make sure that the airline will put you up overnight if the next flight is the next day. Be aware of any connections you have and how they may be affected by being bumped.